1. Technical Field
The present disclosure relates generally to an implant for insertion into a receiving bed formed between adjoining vertebrae. Particularly, the invention relates to an intervertebral implant adapted to fuse with the adjoining vertebrae and including a movement resistant structure for preventing relative motion between the intervertebral implant and the adjoining vertebrae during the period required for fusion.
2. Background of Related Art
Surgical implants are well known in the art for treatment of the spine for deficiencies including disease, trauma, deformity, and/or degenerative spinal conditions. The purpose of the implant is to reinforce and fuse with the spine by use of strategically placed attachment tools or implants. When a segment of the human spine degenerates, or otherwise becomes diseased, it may become necessary to surgically remove the affected disc of that segment, and to replace portions of it for the purpose of obtaining a spinal fusion. The implant primarily functions to restore a more normal, pre-morbid spatial relationships, and provide enhanced stability and support across affected segments.
Generally, implants suitable for intervertebral implantation facilitate fusion of adjoining vertebrae and include movement resistant structures that, add strength and/or prevent expulsion of the implant from the intervertebral space during fusion process.
Intervertebral implants are available in a variety of different shapes including cylindrical dowels, tapered wedges, rectangular blocks, etc. For example, cylindrical dowels may be threaded to retain the implant within the intervertebral space. Alternately, intervertebral implants may include surface ridges, grooves, or protrusions to prevent movement of the implant in relation to the adjoining vertebrae. Structures designed to prevent relative movement between the implant and engaged spinal elements may not always be effective. Thus, spinal fusion procedures may fail due to movement of the implant in relation to the adjoining vertebrae during the fusion process.
There are several approaches for accessing the spinal disc space, typically the spine is approached from the anterior, anterior lateral, lateral, posterior lateral or the posterior direction. The lateral approach is often preferred due to the ease with which the spinal cord, dural sac, major vessels and nerve roots can typically be avoided.
In entering the disc space anteriorly, a very important stabilizing structure, the anterior longitudinal ligament, is compromised. This structure physiologically acts as a significant restraint, resisting the anterior displacement of the disc itself and acting as a tension band binding the front portions of the vertebrae so as to limit spinal hyperextension.
Historically, various devices have been utilized in an attempt to compensate for the loss of this important stabilizing structure. These devices have assumed the form of blocks, bars, cables, plates or some combination thereof, and are bound to the vertebrae by screws, staples, bolts, or some combination thereof. The earliest examples are of a metal plate attached to adjacent vertebrae with course-threaded screws. The following documents illustrate some of the approaches known in the art.
U.S. Pat. No. 4,743,256 discloses the use of a block inserted to replace the disc, affixed to a plate then screwed to the vertebrae above and below.
U.S. Pat. No. 4,401,112 discloses the use of a turnbuckle affixed to an elongated staple such that at least one entire vertebral body is removed, the turnbuckle portion is placed within the spine, and the staple extends both above and below the turnbuckle and engages the adjacent vertebrae to the one removed.
U.S. Pat. No. 6,066,175 discloses a titanium implant assembly having an integrally formed implant and retaining portions.
A unit including separate implant and retaining parts, particularly those made from metal, is so positioned upon its insertion into the intervertebral space so that the retaining portion tends to support a significant portion of spinal loads. Such an uneven distribution of loads causes gradual loosening of the fasteners traversing the retaining portion that attach to the vertebrae.
The retaining portion of known implant assemblies typically has a continuous, flat surface extending complementary to the opposing surface of the spine. But for the fasteners attaching the retaining part to the vertebrae, the retaining part does not have any additional load-bearing surface capable taking loads imposed on the spine. As a consequence, known structures of retaining plates have limited contact areas between the implant and the bony mass of the spine.
A metallic implant always remains a foreign body, which is not able to accurately mimic the biomechanical or biological characteristics of the spine. Although such a metallic implant often consists of an internal graft promoting incorporation and growth of new bone tissue as a result of its osteoconductive capabilities, metallic parts consisting of a cage and a retaining portion do not promote wound healing and/or remodeling of new bone. A large part of the metallic implant never fuses with the adjoining bone and never is replaced by host bone and, thus, will never recover its original, natural qualities. Furthermore, a subsequent surgery is often required to remove the retaining portion of the construct. Since a large area between the metallic implant and the adjacent bone is not capable of fusion, relative motion between the bone and implant may cause gradual loosening of fasteners, which, in turn, leads to undesirable implant mobility. Under certain circumstances such a phenomenon may lead to neural damage, vascular damage and/or bleeding.
Accordingly, there is a need for an improved implant, which allows the implant as a whole to fuse with the adjoining bone and to enable promotion of bone growth. Furthermore, it is desirable to provide an intervertebral implant having more effective movement resistant structure to prevent relative displacement between an intervertebral implant unit and vertebrae during the period required for successful fusion.